As the minimum feature size of the devices used in integrated circuits (ICs) continues to decrease, increased demands are placed on the materials used in the fabrication of these devices. Increased device density and number of interconnect levels in typical ICs have led to a need for materials better suited to achieve the electronic and physical properties necessary for the fabrication of these ICs. Ultra-large scale integration (ULSI) requires more efficient gap filling capabilities of sub-half micron features with high aspect ratios, i.e., the ratio of feature depth to width, which may be as high as 10:1 in some cases. In addition, decreasing the distance between metal interconnect lines requires an inter-metal dielectric (IMD) material, or insulator, with a low dielectric constant in order to minimize parasitic capacitance effects.
Desired properties for the new IMD materials are high reliability, low stress, simplicity of processing and ease of integration. Fluorine-doped silicon oxide films, which are also known as fluorinated silicate glasses (FSG), have been identified as a promising class of low dielectric constant insulators. These FSG materials exhibit enhanced gap filling capabilities over conventional boron or phosphorous doped silicate materials and due to the similarities of FSGs to conventional oxide films, these materials may be integrated into standard IC fabrication processes.
The dielectric constant of FSG materials has been shown to decrease as the fluorine content of the material increases. FSGs with dielectric contents as low as 2.3 have been reported. Shimogaki, Y. et al., "The Contribution of Si--O Vibration Modes to the Dielectric Constant of SiO.sub.2 :F Film," 1996 DUMIC Conference, pp. 36-43. Typical FSGs have dielectric constants in the range of 3.0-3.7. Laxman, R. K., "Low .epsilon. Dielectrics: CVD Fluorinated Silicon Dioxides," Semiconductor International, 1995.
Several methods have been described for the preparation of FSG films. One significant difference in these methods is the choice of fluorine containing precursor. For example, combinations of tetraethylorthosilicate (TEOS), an oxygen source such as N.sub.2 O, O.sub.2 or O.sub.3 and a fluorine source such as C.sub.2 F.sub.6, CF.sub.4, NF.sub.3 and SiF.sub.4 have been used in plasma-based deposition processes, such as plasma enhanced chemical vapor deposition (PECVD) processes. Van Schravendijk, B., et al., "Correlation Between Dielectric Reliability and Compositional Characteristics of PECVD Oxide Films," 1992 VMIC Conference, pp. 372-378; Carl, D., et al., "Comparison of PECVD F-TEOS Films and High Density Plasma SiOF Films," 1995 VMIC Conference, pp. 97-100; Tamura, T., et al., "Preparation of Stable Fluorine-Doped Silicon Oxide Film By Biased Helicon Plasma CVD," 1996 DUMIC Conference, pp. 231-238; Karim, M. Z., et al., "Low Dielectric Constant Materials," Future Fab Int'l, 1996, pp. 213-227. Plasma processing of C.sub.2 F.sub.6 has been shown to generate CF.sub.3 radicals. These CF.sub.3 moieties can undergo further excitation and decomposition to yield atomic fluorine species and these species are then incorporated into the FSG film. SiF.sub.4 also is believed to generate atomic fluorine species under plasma excitation and also results in fluorine species that are substituted into the silicate structure of the deposited film.
Another approach for making FSG films is to incorporate a pre-existing Si--F bond into the CVD precursor molecule. For example, a fluorotriethoxysilane (FSi(OC.sub.2 H.sub.5).sub.3)/pure water system has been described. Homma, T., "Properties of Fluorinated Silicon Oxide Films Formed Using Fluorotriethoxysilane for Interlayer Dielectrics in Multilevel Interconnections," J. Electrochem. Soc., 143(3):1084-1087 (1996). Also, systems using 1,2 bismethyldifluorosilyl!ethane have been described. Yoo, W. S., et al., "PECVD of Fluorine Doped TEOS Oxide Films Using an Alternative Dopant Source," 1996 VMIC Conference, pp. 110-112; U.S. Pat. No. 5,492,736. These materials may be used in conjunction with TEOS and an oxygen source to produce the FSG films.
However, a problem believed to be common to all dual source CVD processes is that variations in the ratio of the precursors may have undesirable effects on the properties of the resulting FSG film. Also, due to the ease of fluoride elimination in perfluorinated alkyl groups directly bound to silicon, perfluoroalkylsilanes are not stable.